Scientific Method —

Maybe we don’t need embryonic stem cells after all

Researchers may have identified a minimal set of genes that can turn any …

Embryonic stem cells (ESCs) have the potential to develop into any tissue, and thus hold promise for repair of damaged organs. Part of that potential comes from being a perfect tissue match to the person in need of repair, but this assumes that ESCs can be made from adults, a process that currently requires using a process that's disturbingly close to human cloning. The alternatives, however, are also problematic. Human ESCs exist, but they will not be perfect matches to patients, and there are restrictions on working with them while using government funding and some ethical concerns regarding their creation. Although adult stem cells exist, they are partly specified, and may not be able to form every tissue that needs repair. In addition, some adult stem cells exist in small populations that reside in hard-to-reach locations—nobody's going to dig around in the heart or interior of the brain of a patient in order to pull out a few stem cells.

In an ideal world, we'd simply convert cells from adult patients directly into stem cells without doing anything resembling cloning along the way. Working in mouse cells, a pair of researchers from Kyoto have apparently done just that. The researchers built on the extensive characterization of stem cells, both human and mouse, that have been performed recently. They first dropped a drug resistance gene into a locus that's expressed in ESCs, so that when cells were cultured with the drug, only those with ESC-like gene expression would survive. Next, they scanned the literature for any gene that is expressed in ESCs, and chose a panel of 24 genes that were known regulators of stem cell formation or embryonic development.

They first introduced these genes individually into mouse cells, but none of the resulting lines were drug resistant. Dumping all 24 in at once, however, produced several cell lines, several of which appeared to be ESCs by a number of assays. The scientists then went through and eliminated one gene at a time from the pool, whittling it down to 10 genes. They repeated this Survivor-like process with the pool of 10, and eventually came up with four genes: Oct3/4, Klf4, Sox2, and c-Myc. Transfection of mouse cells with these four was sufficient to convert them to ESCs.

Gene expression analysis using DNA chips showed that the resulting cells were most similar to ESCs, and no longer resembled the parental cell line. In a number of culture systems, the cells could form a huge range of adult cell types, and could form embryoid bodies when injected into adult mice. But the key test came when they labelled these ESCs with a fluorescent tag and injected then into recently fertilized mouse embryos at a time when the embryos were a small cluster of cells. The progeny of the engineered ESCs glowed green, and were found in every tissue in these embryos as they developed, as well as throughout adults. There seems to be little that's different between regular ESCs and the engineered ESCs.

There are still some question as to what exactly is going on with these cells. The efficiency of conversion to ESCs is very low, but it is unclear what limits it. A second question is why, if these cells still carried the extra copies of these four genes, could they ever differentiate into normal cells? Shouldn't they remain ESCs? The technique is also not ready for use in humans, and not only because it's not been tried with human cells. The technique involved in introducing the genes used retroviruses that inserted randomly into the genome—not generally a safe technique. Still, this appears to be an important first step, and you can bet that many labs will be interested in following up on these results.